The Roots blower or compressor is widely known and is named after an early patentee of the apparatus (U.S. Pat. No. Re. 2,369). They have been primarily used for moving large quantities of air at low pressures with compression ratios of less than 2.
A Roots blower generally comprises a housing having an inlet and outlet port and walls defining a cavity therein. The cavity is enclosed and shafts inserted therein for carrying lobed impellers between the inlet and outlet ports and caused to rotate in opposite directions within the cavity. The rotation of the impellers is synchronized, by the utilization of timing gears, so that a predetermined clearance is continually maintained between the impellers as they rotate into and out of register with each other. Gas is introduced through an inlet port, pulled into the chamber by the motion of the impellers, then compressed within the cavity and exhausted to an outlet port.
One application for a Roots blower has been in a vacuum swing adsorption process for the separation of nitrogen from air as illustrated in U.S. Pat. No. 4,013,429 which is incorporated by reference. In this type of process, air is passed serially through a pretreatment adsorbent bed for removing moisture and carbon dioxide and then through a main nitrogen adsorbent bed. Once the main bed is fully adsorbed, the nitrogen is desorbed by suction established by the Roots blower and then the bed rinsed with a high purity nitrogen. In the vacuum swing system for obtaining a high purity nitrogen product and an oxygen-enriched gas fraction, the Roots blower is subjected to wide pressure oscillations within Very short periods of time. Typically, the pressure range within the vacuum swing adsorption system ranges from 1 to 2 pounds psig to a vacuum of 10 to 50 Torr. The beds are cycled from high pressure to low pressure generally from every 2 to 8 minutes.
Two problems were presented with respect to the Roots blower by the process conditions imposed in the vacuum swing process for the recovery of nitrogen from air. First, dust which resulted from attrition or comminution of the zeolite in the adsorbent bed would migrate into the cavity of the Roots blower during desorption of the bed. This dust would remain in the cavity and then on recompression of the bed the dust in the cavity often would be forced into the bearing compartment by virture of the pressure differential between the cavity and bearing compartment. The dust when combined with lubricant then would cause extreme wear and early failure of the bearings. Second, lubricant, in time, would migrate from the bearing compartment into the cavity and contaminate product gas. The migration of the lubricant occurred during desorption and resulted because of the pressure differential between the bearing compartment and blower cavity. Once lubricant was in the cavity it was then exhausted with product gas or into the zeolite bed.
Numerous attempts were made to solve the sealing problem in the Roots blowers. One technique was to pull a vacuum on the bearing compartment to a level below any pressure that might exist in the cavity. Although the process was effective for preventing lubricant migration into the cavity dust contamination appeared in the seal rings and the bearings were rapidly destroyed. Another technique for effecting a seal was to utilize a multiple series of mechanical seals and purge; e.g., a plurality of seal rings with a nitrogen purge between the inner and outer set of the sealing rings. However, problems were experienced with this particular system in that it resulted in large losses of product nitrogen or the seals were difficult to fabricate.